This invention relates to variable capacity air conditioning compressors in general, and specifically to such a compressor with improved crankcase oil retention.
Piston driven automotive air conditioning compressors with variable capacity generally vary the piston stroke by allowing the angle of a nutating piston driving plate to change relative to the centerline of a drive shaft. Smaller angles yield a shorter nutation and shorter piston stroke, and larger angles create a loner piston stoke. The tilting plate may be of the unitary type that directly drives the pistons (swashplate), or a compound type that indirectly drives the pistons (wobble plate). In either case, a plate tilt mechanism consists of several sliding and pivoting members located behind the pistons and within the main hollow body of the compressor housing, the so called xe2x80x9ccrankcasexe2x80x9d volume. All rubbing interfaces within the compressor and the crankcase, including the tilt mechanism, require sufficient lubrication for proper operation, and this depends on lubricant being carried to different parts of the compressor by the refrigerant in which it is entrained. To the extent that lubricant is well retained within the crankcase, these sliding interfaces are well lubricated.
The compressor pumping capacity can be controlled by allowing the plate to shift to a different angle, rather than externally physically moving it along the shaft. This is done by controlling the net pressure differential between the front or head of the pistons and the rear of the pistons. The back of the pistons face the inner volume or crankcase, while the heads of the pistons face the pressure in a suction chamber, and the two pressures between which the differential exists can be referred to as crankcase and suction pressure. When there is substantially a zero crankcase-suction pressure differential, there is no net resistance preventing the piston from moving back as far as it can, so that the plate is allowed to shift to its largest angle relative to the shaft centerline, creating the longest piston stroke. At the highest pressure differential, there is the highest net resistance to the piston backstroke, so the plate shifts to the smallest angle relative to the shaft centerline, creating the shortest stroke of the piston.
A capacity control valve in the compressor body controls the net pressure balance on the piston by controlling refrigerant gas flow into or out of the crankcase. The valve can be responsive to both suction pressure and discharge pressure to control selective communication of compressor discharge and suction chambers with the crankcase, thereby controlling the net pressure balance on the pistons (and thereby controlling the effective piston stroke and capacity). The controlled refrigerant flow requires the provision of a flow passage for gas flow from the crankcase to the control valve and ultimately to the suction chamber, and, in swashplate compressors, such crankcase to suction passages typically been bored through the back of the cylinder block, the structural member in which the piston cylinders are formed. This is further described below in the description of FIG. 1. As such, the inlet opening of the crankcase to suction cavity passage flow passage has been directly and clearly exposed to the crankcase, and thereby directly exposed to the greatest swirl and velocity of refrigerant gas. Oil in the compressor which would otherwise be retained can be easily blown out. In a wobble plate compressor, the equivalent crankcase to valve to suction chamber flow passage is bored through the central shaft and part of the plate tilt mechanism, with the inlet opening to the passage located even more deeply into the crankcase volume and even more exposed.
The subject invention provides a variable capacity compressor in which the initial portion of the crankcase-to control valve-to suction chamber passage is bored through the central drive shaft, rather than through the cylinder block, but in which the inlet opening is not exposed directly to the main portion of the crankcase. Instead, the inlet opening is sheltered within a central cylinder block bore, inset from the plane of the back of the cylinder block, and therefore isolated from the more turbulent main portion of the crankcase. Within the sheltered and isolated volume surrounding the inlet passage, the refrigerant is less turbulent, carries less entrained lubricant, and, therefore, less lubricant is forced out of the crankcase with the flow of refrigerant through the passage.